1. Field of the Invention
The present invention relates, in general, to a shuttle bobbin used for carrying the lower thread in sewing machines, the lower thread meeting the upper thread so as to make a stitch on cloth, and, more particularly, to a shuttle bobbin with the thread winding hub being partitioned into two or more parts having different thread winding capacities, thus allowing the lower thread to be wound on the bobbin in a multi-step winding style, and enlarging the thread carrying capacity of the shuttle bobbin, and thereby lengthening the interval of changing an empty bobbin with a new one while improving work efficiency while performing a sewing operation.
2. Description of the Prior Art
As well known to those skilled in the art, sewing machines are designed to use a shuttle bobbin, which carries a lower thread to meet an upper thread, making a stitch on cloth. A typical shuttle bobbin for sewing machines is shown in FIG. 5. Such a known shuttle bobbin "a" is removably set in a sewing machine at an appropriate position under the sewing plate, thus allowing the lower thread to be continuously fed to the cloth on the sewing plate while performing a sewing operation. In order to continuously feed the lower thread to the cloth while performing a sewing operation, it is necessary for a user to repeatedly and periodically change an empty bobbin with a new, full bobbin of thread.
Such a shuttle bobbin is removably received in a bobbin case prior to removably installing the bobbin case in a sewing machine at an appropriate position under the sewing plate. The size or the thread carrying capacity of the shuttle bobbin is thus determined in accordance with the internal volume of the bobbin case. However, the external size of the bobbin case is internationally standardized, so that the internal volume of the bobbin case is limited. Therefore, it is almost impossible to freely enlarge the thread carrying capacity of a shuttle bobbin without being limited, so that the design of such a shuttle bobbin is limited.
In other words, the internal volume of conventional bobbin cases is limited due to the desired strength in relation to the thickness of the walls of the internationally standardized bobbin cases. In addition, the projection of the shuttle bobbins from the bobbin cases is limited to an extent free from causing any interference with a sewing needle in its lowest position, so that the size of the shuttle bobbins is limited.
In an effort to enlarge the thread carrying capacity of a shuttle bobbin while considering the above-mentioned structural limit of both the shuttle bobbins and the bobbin cases, the bobbin cases are designed to have an enlarged inner diameter which allows a shuttle bobbin to carry more thread. In such a case, it is necessary to shave the interior surface of a bobbin case so as to enlarge the inner diameter of the bobbin case suitable for receiving a shuttle bobbin, carrying more thread. However, the increase in the inner diameter of the bobbin case is structurally limited and this limits the increase in the thread winding height of the shuttle bobbin. Therefore, the bobbin cases, of which the interior surfaces are shaved so as to increase the inner diameter of the cases, fail to effectively enlarge the thread carrying capacity of shuttle bobbins. It is thus necessary for a user to repeatedly and periodically change an empty shuttle bobbin with a new, full bobbin of thread while performing a sewing operation. This reduces work efficiency while performing a sewing operation and increases the sewing cost.